Q24 · 2025 · Paper 1Simple familiar9 marks
QUESTION 24 (9 marks)
A monochromatic light source was shone on a metal surface and photoelectrons were ejected. A different light source of lower frequency was then shone on the same metal surface. No photoelectrons were ejected, even when the intensity of the light was increased.
a)[5 marks]
Explain how this scenario provides evidence of the particle nature of light and contradicts the concept of light as a wave. In your response, refer to how photoelectrons are affected by the intensity, frequency and energy of light.
b)[4 marks]
A photoelectric experiment used light of frequency , and the maximum kinetic energy of the photoelectrons ejected from the metal was measured to be .
Calculate the work function in electron volts of the metal used in this experiment. Show your working.
WORKED SOLUTION
9 marksQCAA guide · typeset solution
ANSWER
See worked solution below.
Worked solution
a) Electrons need to absorb a certain amount of energy to be ejected. If light acted as a wave only, as intensity of the light increased, electrons would eventually collect enough energy and photoelectrons would be detected. When light is thought of as a particle, increasing the intensity of light increases the number of particles. Since , the light needs to have a minimum frequency for the photons to have enough energy to give to the electrons.
b)
Therefore work function =
recognises electrons need energy to be ejected
recognises the lack of evidence of wave nature of light
recognises the relationship between the photon’s energy and its frequency
explains how the particle nature of light provides evidence for the relationship between the intensity of the light and the number of particles
explains the nature of the threshold frequency
recognises that the scenario relates to the kinetic energy of the photoelectric effect
provides appropriate mathematical reasoning
converts work function units
calculates work function of the metal
One QCAA sample method typeset for web; criterion wording is adapted from the official marking guide.
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